1. Field of the Invention
This invention relates to a display device and, more particularly, to a spontaneous emission-type display device such as organic electroluminescense device.
2. Description of the Related Art
A device using electroluminescense of an organic material (hereinafter called xe2x80x9corganic EL devicexe2x80x9d) comprises an organic layer formed by stacking an organic hole transport layer or organic light emitting layer between a first electrode and a second electrode, and it is remarked as a spontaneous emission type display device capable of high-luminance emission by low-voltage direct current driving.
FIG. 1 shows a cross-sectional configuration of a central part of a transmission-type organic EL device as one of such organic EL devices. The organic EL device shown in FIG. 1 is made by stacking on a transparent substrate 1, sequentially from bottom to top, a transparent electrode 2, organic buffer layer 3, organic hole transport layer 4, organic light emitting layer 5 and metal electrode 6, such that light h generated in the organic light emitting layer 5 is extracted through the substrate 1.
In the organic EL device shown in FIG. 1, however, spectrums of respective colors in the light h generated in and extracted from the organic light emitting layer 5 having various emission colors have wide peak widths as shown in FIG. 2, and especially regarding the red light h, the peak wavelength is deviated to a lower wavelength. Therefore, with a display apparatus using such organic EL devices for color representation, it was impossible to obtain a sufficient color reproducibility range enough to display, for example, TV images.
To overcome this problem, it was proposed to interpose a dielectric mirror layer (not shown) between the substrate 1 and the transparent electrode 2 such that the dielectric mirror layer, transparent electrode 2, organic buffer layer 3, organic hole transport layer 4, organic light emitting layer 5 and metal electrode 6 form a cavity structure. In the organic EL device having this cavity structure, the light h generated in the organic light emitting layer 5 reciprocates between the dielectric mirror layer and the metal electrode 6, and only a part of the light having the resonant wavelength is extracted through the substrate 1. Therefore, light h having high peak intensity and a narrow spectrum can be extracted, and it is possible to enlarge the color reproducibility range of a display apparatus using such organic EL devices.
However, as the peak width of the spectrum of extracted light h is narrowed, as it is in the organic EL device having the above-mentioned cavity structure, dependency of the emission property on the view angle increases. For example, when the emission surface is viewed from an oblique direction, wavelength of the light h largely shifts and the emission intensity decreases. Therefore, it is important that the spectral width of the light extracted from the organic EL device is not narrow too much. However, the organic EL device mentioned above did not contemplate the dependency on the view angle in its design and could not maintain a sufficient color reproduction range over wide view angles.
Such dependency on the view angle is a serious problem especially for white that is very perceivable visually. FIG. 3 shows an example of calculation of view-angle dependency of the chromaticity of white. For liquid crystal or other type displays, allowable range of color deviation in the screen is approximately within xcex94uv=0.015. It is certainly as small as xcex94uv=0.006 when the view angle is 30xc2x0 as shown in FIG. 3. However, when the view angle is 60xc2x0, however, it becomes as large as xcex94uv=0.0178. That is, it is apparent that the color deviation becomes very large beyond the allowable range. This calculation was carried out under the condition where peak wavelengths of multiple interference filter spectrums shown in FIG. 4 and peak wavelengths of internal emission spectrums shown in FIG. 5 by broken lines (spectrums of light emitted by light emitting layers and extracted without multiple interference) (more precisely, the peak wavelengths are those maximizing the efficiency of utilizing the light energy). In this case, the light energy extracting efficiency is maximized. Multiplication of the internal emission spectrums and the multiple interference filter spectrums results in the extracted light having the spectrums shown in FIG. 5 by solid lines.
It is therefore an object of the invention to provide a spontaneous emission type display device capable of adjusting the RGB balance in a luminance-variable quantity under the existence of a view-angle dependency and capable of alleviating the view-angle dependency of white.
According to the first aspect of the invention, there is provided a display device in which a light emitting layer is interposed between a first electrode and a second electrode, and at least one of the light emitting layer and one of the first and second electrodes from which light is extracted functions as a cavity portion of a cavity structure for resonating light generated in the light emitting layer,
characterized in that the peak wavelength of internal emission spectrum inside the light emitting layer and the peak wavelength of multiple interference filter spectrum by the cavity portion are shifted from each other.
According to the second aspect of the invention, there is provided a display device in which a light emitting layer is interposed between a first electrode of a light reflective material and a second electrode of a transparent material, and at least one of the second electrode and the light emitting layer functions as a cavity portion of a cavity structure for resonating light generated in the light emitting layer,
characterized in that the peak wavelength of internal emission spectrum inside the light emitting layer and the peak wavelength of multiple interference filter spectrum by the cavity portion are shifted from each other.
According to the third aspect of the invention, there is provided a display device in which a first electrode of a light reflective material, a light emitting layer and a second electrode of a transparent material are sequentially stacked on a substrate, and at least one of the second electrode and the light emitting layer functions as a cavity portion of a cavity structure for resonating light generated in the light emitting layer,
characterized in that the peak wavelength of internal emission spectrum inside the light emitting layer and the peak wavelength of multiple interference filter spectrum by the cavity portion are shifted from each other.
In any of those aspects of the present invention, in the case where the phase shift occurring upon reflection of light generated in the light emitting layer at opposite ends of the cavity is "PHgr" radians, the optical distance of the cavity is L, and the peak wavelength of the spectrum of the light to be extracted from the light generated in the light emitting layer is xcexmax, the display device is configured to satisfy Equation (1) below.
2L/xcexmax+"PHgr"/2xcfx80m (m is an integer)xe2x80x83xe2x80x83(1)
In the display device having that configuration, since the optical length L of the cavity satisfies Equation (1), light near the wavelength xcexmax runs to multiple interference in the cavity.
In the present invention, color deviation xcex94uv of white upon the view angle of 60xc2x0 is held not to exceed 0.015 by shifting the peak wavelength of the internal emission spectrum in the light emitting layer and the peak wavelength of the multiple interference filter spectrum by the cavity from each other.
The present invention having the above-summarized configuration can adjust the RGB balance in luminance variable-quantity under the existence of a view-angle dependency by means of a deviation of the peak wavelength of the multiple interference filter spectrum relative to the peak wavelength of the internal emission spectrum.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.